High capacitance variable capacitor

ABSTRACT

IN A LAMINATED STRUCTURE, ONE ELECTRODE IS DISPOSED ON A THIN CERAMIC DIELECTRIC MEMBER IN OVERLYING RELATION TO A LOWER ELECTRODE, AND INCLUDES A PLURALITY OF SPACED CONDUCTIVE ISLAND AREAS WHICH ARE CAPABLE OF BEING INTERCONNECTED IN PARALLEL BY A MOVABLE FLEXIBLE WIPER THAT PROVIDES ELECTRICAL CONNECTION BETWEEN THE ISLANDS AND AN ADJACENT TERMINAL OF THE CAPACITOR.

United States Patent Ilohn H. Fabricius Inventor Stamford, Vt.

Appl. No. 880,529

Filed Nov. 28, I969 Patented June 28, 1971 Assignee Sprague ElectricCompany North Adams, Mass.

HIGH CAPACITANCE VARIABLE CAPACITOR 10 Claims, 7 Drawing Figs.

US. Cl. 317/249, 3 l 7/249R Int. Cl. Il0lg 5/08 Field of Search 317/249,249-(D) [56] References Cited UNITED STATES PATENTS 2,789,259 4/1957Eisler 317/249 2,790,970 4/1957 Kodama 3 l 7/249X Primary Examiner-45.A. Goldberg Attorneys-Connolly and Hutz, Vincent l-I. Sweeney, JamesPaul O'Sullivan and David R. Thornton ABSTRACT: In a laminatedstructure, one electrode is disposedon a thin ceramic dielectric memberin overlying relation to a lower electrode, and includes a plurality ofspaced conductive island areas which are capable of being interconnectedin parallel by a movable flexible wiper that provides electricalconnection between the islands and an adjacent terminal of thecapacitor.

BACKGROUND OF THE INVENTION This invention relates to high capacitancevariable capacitors, and more particularly to a high capacitancevariable ceramic capacitor having enhanced stability.

The prior art I impediment, to reaching stable higher capacitance valuesin variable ceramic capacitors has been the heretofore unavoidablepresence of a low dielectric constant medium in the form of an interfacebetween the movable electrode and the ceramic dielectric. This gap,whether it be air or liquid, has a much lower dielectric constant thanthe ceramic dielectric body material separating the capacitor electrodesand thereby constitutes an undesirable capacitance in series with thebody capacitance.

An object of this invention is the provision of a variable capacitor inwhich the interface gap is eliminated.

Another object is the provision of a variable capacitor having highcapacitance with a high degree of stability.

Still another object is to provide a laminated monolithic capacitancemember of variable capacitance.

A further object is to provide a movable contact of large surface areahaving a plurality of interconnected contact areas capable of providingsubstantially independent and uniform contact pressure to adjacentpoints over a wide area.

SUMMARY OF THE INVENTION In accordance with this invention a highcapacitance variable capacitor employs a layered structure having a pairof fixed electrodes separated by a thin dielectric film, and a contactfor providing connection between isolated areas of one electrode and aterminal thereof.

In general, the variable high capacitance capacitor of this inventionhas both electrodes and their output terminals arranged in a laminatedmember with circuit contact to isolated areas of an exposed surfaceelectrode being made through another member which provides a bridgingconnection between one terminal and the isolated electrode areas.

In a preferred embodiment, a continuous electrode and its terminal aredisposed on and insulative substrate and covered with an overlying thinfilm of dielectric material. Another terminal and a discontinuouselectrode, comprising a plurality of conductive elements or islands, areprovided in a coplanar arrangement on the upper surface of thedielectric material with the island electrode overlying the lowerelectrode and forming a capacitance in cooperation therewith. Change incircuit capacitance is obtained by making substantially ohmic contactbetween an incrementally increasing or decreasing number of theconductive islands and the adjacent terminal.

This ohmic contact is made, in the preferred embodiment by a rotatableresiliently mounted fine mesh wire gauze which provides, duringrotation, a continuous intimate contact between all underlying islandsand the adjacent terminal. This arrangement provides a capacitor which,by eliminating the interface gap and by use of high K dielectric filmsof very few mils) mils thickness, increases maximum stable capacitanceby several orders of magnitude over conventional variable capacitors.

In one particular embodiment illustrated herein, the island electrode isshown as a pattern of square islands placed on the dielectric surface.The islands are interconnected in parallel to their terminal by arotatable contact member, and their arrangement is designed so that therotating wiper makes incremental ohmic contact with as small a number ofislands as possible during its advance. In another embodiment, theisland electrode is formed in several zones, each having a differentisland geometry. It is also possible to form a multiplicity ofmicroscopic islands by depositing a noncontinuous conductive film on thedielectric surface.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a plan view of the stator ofthe preferred embodiment of the present invention showing the islandelectrode pattern and its terminal on an upper surface;

FIG. 2 is a cross-sectional view of the stator taken along line 2-2 ofFIG. I;

FIG. 3 is a plan view of the rotor utilized in the preferred embodiment;

FIG. 4 is a cross-sectional view of the rotor taken along line 4-4;

FIG. 5 shows the assembled capacitor provided in accordance with apreferred embodiment of the present invention;

FIG. 6 is a plan view of an island electrode illustrating another islandpattern; and

FIG. 7 is a view in section of an alternate stator structure whereineach outer major surface has an island electrode thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 illustrate astator member 11 wherein a thin (0.2 mil) film electrode IZof conductivematerial, such as silver or the like, is deposited as a generallysemicircular annular segment on a portion ofa major surface ofinsulating substrate l3. Substrate 13, which operates as the supportlayer of the laminated stator, has an aperture 14 therethrough,substantially located at the center or radius of conductive pattern I2.Substrate 13 is of sufficient rigidity and thickness, for example, 0.030inch in the preferred embodiment, to fixedly support the overlyinglayers. A terminal 15, which is formed with and of the same material aselectrode I2, extends from the circumference of electrode 12 to providea terminal pad to which a circuit lead can later be soldered.

FIG. 2 shows a layer 16 of a ceramic dielectric deposited over thesurface of film l2 and surrounding portions of substrate IS; thethickness of layer I6 overlying film I2 being approximately l mil. Forthe present embodiment, aluminum oxide has been used for the supportingsubstrate and a barium titanate glass mixture having a dielectricconstant of approximately 400 for dielectric layer 16. For example, thebarium titanate mixture as described in U.S. Pat. application No.767,046 filed by Galeb A. Maher on Sept. 26, I968 is suitable. A thinfilm (0.7 mil thick) of conductive material such as silver or the like,is then deposited over the outer surface of layer 16 in an electrodepattern 17, having islands 18, and a connective portion which includescontact area 19 and input terminal 20. Pattern I7 is disposed over andcooperates with the underlying continuous electrode 12 to form acapacitance.

Various methods of deposition for formation of the continuous and theisland films can be used, such as spraying, sputtering, dipping, vacuumdeposition and silk screening. Of course, the isolated electrode areascan be fonned during film deposition or may be formed from a completedcontinuous film by photolithographic etching techniques or the like.Some of the appropriate materials for these conductive films aretantalum, gold, silver, platinum, rhodium, iridium, copper, molybdenum,or combinations thereof.

FIGS. 3 and 4 illustrate rotor member 23 of the present invention havinga central aperture 24 therethrough. Member 23 comprises a flat washer 25to which is affixed a resilient pressure pad or washer 26 for example,of silicon rubber or the like is preferable, since it resists pressureset. A conductive wiper 27 of wire mesh gauze is attached to one-half ofthe outer surface of washer 26 while an insulating skid plate 28 isaffixed to the other half. The gauze can be 250 mesh square weave orfiner and may be of brass, phosphorous bronze, stainless steel or otherconductive material. Acceptable materials for skid plate 28 are Teflonor other films exhibiting a low coefficient of friction. For example,nylon or polyethylene film are also suitable for this purpose.

A completely assembled capacitor unit is shown in FIG. 5. whereinrotating member 23 is securely mounted on mating segment 31 of shaft 32.Rotor 23 is keyed to or affixed to shaft segment 31 by, for example, keyslot 30. One end of shaft 32 passes through central aperture 24 of rotor23 and also projects through aperture 14 of stator 11. Stator ii androtor 23 are installed so that their respective surfaces containingpattern 17 and wiper gauze 27 are in abutting relation. These abuttingsurfaces are biased into engagement by pressure spring 33 mounted on theend of shaft 32. The spring 33 acts against buffer element 34, which isa plastic washer or the like to force stator 11 and rotor 23 towards ashoulder 29 of shaft 32. Terminal leads 35 and 36 are connected toterminal areas 15 and respectively.

Assuming a signal is present at terminal leads 35 and 36 to which aspecified capacitance value is to be presented, and further assuming forpurposes of the example that a clockwise rotation of shaft 32 (and rotor23) is utilized, as wiper 27 and skid plate 28 begin to slide over theabutting face of stator member 11, wiper 27 engages islandic pattern 17beginning at point 37 (HO. 1) and provides a bridging connection tocontact portion 19 (and terminal 20). As indicated, each island area 18of electrode 17 acts with an opposing portion of electrode l2 (anddielectric l6 therebetween) to provide capacitance. Hence, as wiper 27penetrates further into engagement with pattern 17, each island 18 isincrementally and electrically connected in parallel with precedinglycontacted islands and contact area 19 to provide a capacitance atterminals 15, 20 in accordance with the area of the contacted islands.The rows of island areas forming pattern 17 are generally locatedobliquely to the leading edge 39 of wiper 27, that is, offset from thecenter of the annular segment so as to allow wiper 27 to contact anincrementally increasing number of islands 18 during rotation so as toassure the smoothest capacitance change.

The pressure exerted by spring 33 (between 10 and l00 p.s.i.) throughpressure equalization pad 26 is sufficient to press wire gauze 27uniformly and intimately against each conductive segment. Insulatingskid plate 28, also mounted on washer 26, imparts a smooth rotation tothe rotor member by moving over the stator surface with a minimum offriction.

Advantagcously, the capacitor terminals are mounted directly on stator11 which permits simplified external connection. Additionally, bothisland electrode 17 and its terminal contact area 19 are exposed on oneside of the stator such that mechanical biasing of the rotor towardsstator 11 simultaneously provides both electrode and terminal contact.For simplicity of construction electrode 17 and its terminal contactportion 19 should lie in substantially parallel planes, and preferablybe substantially coplanar.

It is also preferable that complete annular stator and rotor surfaces beengaged. That is, electrode 17 and its contact terminal area 19preferably form a full circle on which rides a full annular rotor,comprising substantially coplanar wiper 27 and skid 28. Of course, acomplete annular ring may be provided in many ways, for example,electrode 17 and contact area 19 may be arranged so as to be contiguousalong at least portions of their curved perimeters and a nonskidmaterial be utilized to provide a complete stator annulus. in this case,wiper 27 would be increased along the radius (widened) to reach both theelectrode and contact area.

As an added refinement, an insulative film 21 of low dielectric constantmaterial, for example, a silicon varnish or a polyimide or the like maybe deposited between islands 18 so as to provide enhanced operation.Film 21 is provided to a thickness equal to or slightly less than islandheight. Film 21 fills the separation between islands and serves furtherto stabilize the capacitor by reducing interisland capacitance whichcould cause undesirable capacitance variations with minor changes inrotor position or interface pressure. in this regard, film 21 not onlyprevents foreign material from filling the separation between islands,but also prevents any portion of wiper 27 from approaching the highdielectric material at the bottom of the separation. Hence, film 21prevents wiper 27 from becoming part of or adding to the overall area ofelectrode 17 so that only the connection of particular capacitiveportions to the circuit terminals is varied, and the actual capacitanceof stator 11 never changes. Teflon or other low friction material canalso be employed as film 21 so as to increase stability and reducefriction.

The wiper rotation can be stopped at any desired point up to its fullcapacitance position (when all areas of the islandic pattern 17 areengaged by wiper section 27) to establish a desired capacitance value.inasmuch as contact 19 abuts the end of the annular segment making upelectrode 17, and since the latter is of slightly shorter length thanwiper 27, (see edge 37) electrical contact of wiper 27 is maintainedwith a diminishing portion of contact area 19 during rotation but evenin full capacitance position, is still in electrical contact with atleast a portion of this surface and maintains electrical continuitytherewith. As rotation continues past the full capacitance position, oras rotation is reversed, the capacitance begins to decrease as islandconnection is reduced.

Wire gauze wiper 27, which is used to contact islands 18, providesuniform contact resistance, and eliminates undesirable effects such asnoise and hysteresis of capacitance versus angular shift rotation. Thesedefects, prevalent in the prior art, are reduced or eliminated by use ofthe mesh type gauze wiper which, when a capacitance position isestablished, relaxes and clings to the electrode segments establishing auniform and intimate contact between all previously contacted segments.While the 400 mesh square weave of 0.7 mil wire proved to be optimum,other weaves such as twill and different size mesh may be used.

The mesh combines high conductivity,high flexibility, good wearresistance and an appropriate surface roughness to provide a novel longlife sliding contact which is in essence a multiplicity ofinterconnected but substantially independent spring loaded (by rubberwasher 26) contact areas having a few mils separation. it also includesa foraminous surface which provides multiple reservoirs or cavities forretaining undesirable materials automatically scoured from the electrodeface during rotation. A foraminous metal film, a few tenths of a mil inthickness may be alternatively employed, however, this would not be asflexible or contribute the sinuous mesh surface.

As illustrated, rotor 23 is a laminated structure having its layersaffixed to each other, however, in an alternative arrangement only wiper27 and skid plate 28 are joined together and pad 26 is sandwichedbetween this assembly and washer 25 solely by mechanical bias applied inthe completed capacitor unit. Where pad 26 provides a surface havinghigh friction, for example, where it is of rubber material, mesh 27 willsufficiently track with pad 26 and washer 25, during rotation of thelatter. However, to insure tracking, wiper 27 can also be keyed to oraffixed to shaft 32 which drives washer 25, and indeed, all the separateelements making up the rotor can be so affixed to rotor shaft 32.

As an alternative wiper, which provided satisfactory results, aconductive rubber strip of metal loaded rubber was employed for contactwiper 27. This wiper configuration also has sufficient resiliency tomake a highly uniform contact to selected islands, Additionally, it maybe directly seated on flat washer 25 so to provide both conductive wiper27 and rubber pressure washer 26.

For maximum efficiency the distribution of islands 18 should providemaximum effective use of the capacitance surface. Hence the geometry ofeach individual island segment is important. For example, assume thatthe outer diameter D of the stator annulus is 0.5 inch and the innerdiameter d (of aperture 14) is 0.05 inch, while each island segment 18is a square having 0.008 inch sides, and a 0.002 inch space betweenislands. Then, the distance b from one island edge to the next adjoiningisland edge will be 0.010 inch and the percentage effective area coveredby the island segment is The total area available for pattemdistribution (assuming electrode 17 is a semicircle) is and theeffective electrode area is then 0.062 inch Then in the given example,the maximum available capacitance in picofarads is using a dielectriclayer 16 having a higher dielectric constant.

For example, barium titanate having an excess of titanium provides adielectric constant of 3000. This produces a capacitance ofapproximately 42,200 pf. in the above example.

Although'the island pattern of the preferred embodiment comprises squaresegments, the invention is not limited to such a pattern. The islandsegments, may be rectangular, round, oval, etc. Moreover, the patternneed not be uniform throughout as evidenced in FIG. 6 which shows analternate embodiment of pattern 17 of FIG. 1.

In FIG. 6, the substantially 180 of surface rotation available forpattern 17 is divided into four sectors or quadrant zones 17a through17d, each of which has a different row orientation and islands ofdiffering geometry. In this case, islands 18 increase in size in eachsucceeding quadrant. This arrangement affords a capacitance variation atdifferent incremental rates depending upon the sector over which wiper27 is advancing. For example, as wiper 27 advances into quadrant 17a(assuming a clockwise rotation), islands 18a into which zone 17a issubdivided, are relatively small and the incremental capacitance changeis relatively small, and wiper 27 advances into quadrants 17b, c and dit begins to add (to the previously formed capacitance) the relativelylarger island segments 18b, 18c and 18d. A typical distribution would beas follows:

quadrant 17a 43 arc and 56 percent effective area quadrant 17b 50 arcand 66 percent effective area quadrant 17c 45 arc and 76.5 percenteffective area quadrant 17d 42 arc and 80 percent effective area withthe total effective electrode area amounting to 70 percent.

Special rates of capacitance variation with angular rotation (or slidingrate of wiper 27) may also be achieved by varying the geometry ofelectrode 12. Hence, instead of a regular annular segment as described,electrode 12 may vary inwidth etc. For example, the width of thesemicircular strip that makes up electrode 12 may decrease from one endto the other so as to provide a particular capacitance curve (even withuniform islands 18) as wiper 27 progresses in this direction. It beingunderstood of course, that capacitance variation achievable byvariationin the overall geometry of pattern 17 would also be possiblebut would permit undesirable effects due to the wiper contact with thesurface of dieleccould be provided in a peripheral arrangement.Additionally, other variable components, such as resistors and inductorsas well as fixed capacitors, resistors and inductors could be included,for example, the fixed element may be disposed between any terminal andits electrode.

Other embodiments of this invention consistent with the above describedfeatures may also be practiced. For example, the available capacitancemay be increased by making use of both sides of stator 11 as shown inFIG. 7 wherein the crosssectional structure shown in FIG. 2 is modifiedby depositing another continuous electrode 40 on the opposing surface ofsubstrate 13, and thereafter, forming a second dielectric layer 41 overelectrode 40. Then, another conductive film is formed on the outerdielectric surface with a portion overlying electrode 40 formed intoelectrode pattern 42, having a plurality of islands 43, while theremaining portion is formed into a second contact area 44 and an inputterminal (not shown). Pattern 42 forms a fourth electrode and, with theunderlying portion of third electrode 40, comprises a second variablecapacitor. A second low dielectric film 45 may also be formed betweenislands 43. Assuming the same materials, surface areas and patterngeometry etc. of the first stator capacitor as described above, a secondwiper identical to that previously described in FIGS. 3 and 4 willresult in substantially the same capacitance as it opposite member as itadvances across islandic pattern 42 in synchronism with the first wiper.If the capacitance of both capacitors are connected in parallel, thetotal available capacitance will thereby be doubled.

The two wipers can also be rotated in opposite directions or out ofphase with each other, for example, the upper wiper may be mounted onshaft 32 so that it is keyed l80 from the lower wiper. In this case, andassuming each continuous electrode (electrodes 12 and 40) is connectedto a different potential and both terminals of the islandic electrodesare commonly connected, the capacitor pair acts as a capacitance voltagedivider. A signal potential can then be developed intermediate that oneither electrode. The voltage divider can more easily be constructed bytwo semicircular segments formed on substrate 13 in contact withrespective terminals, and a discontinuous annular ring overlying bothand adapted for rotor contact to a third terminal.

Advantageously, stator 11 provides a variable capacitor member in alaminated arrangement in which a support member provides a conductiveelectrode surface which carries a thin dielectric member and anoverlying discontinuous tric film 16 in areas overlying electrode 12. Ofcourse, this electrode so as to provide exceptionally high capacitancewith no interface gap. In the preferred embodiment, substrate 13 is ofinsulative material having a conductive film l2 and dielectric film 16deposited on its overlying surface, however, other arrangements arepossible. For example, electrode 12 and film 16 may consist of a thintantalum sheet having an oxidized upper surface, or of reduced bariumtitanate having a thin oxidized surface.

In appropriate cases, the substrate itself may be of conductive orsemiconductive material such as tantalum or reduced barium titanate. Ofcourse, if the substrate body is to be of conductive material, contactarea 19 and its terminal 20 have to be substantially isolated from thesubstrate by low dielectric constant material or the like in order toavoid their contribution of capacitance in this case.

I claim:

1. A high capacitance variable capacitor comprising a stator includingan insulative substrate having a conductive surface portion providing afirst electrode, a thin dielectric film overlying said first electrode,a discontinuous second electrode disposed on said film in overlyingrelation to said first electrode, said second electrode including aplurality of isolated conductive island areas, a first terminal and asecond terminal respectively located adjacent said first electrode andsaid second electrode, said first terminal contacting said firstelectrode; and a rotor having a resilient conductive surface portionbridging between said second terminal and selected of said island areasfor providing a capacitance between said first and second terminals inaccordance with the rotation of said rotor into contact with anincrementally changing number of said island areas.

2. The capacitor of claim I wherein said second electrode and secondterminal are substantially coplanar members exposed on a surface of saidfilm, and said conductive surface portion of said rotor is asubstantially planar member adapted to slidably engage said secondterminal and said second electrode thereby bridging between said secondterminal and said selected island areas.

3.'The capacitor of claim 2 wherein said slidable contact memberincludes a resilient layer having a highly flexible conductive surfaceportion providing a multiplicity of interconnected contact areasproviding substantially uniform contact pressure to adjacent islandareas.

4. The capacitor of claim 1 wherein separations between said islandareas are substantially filled with solid dielectric material having amuch lower dielectric constant than that of said dielectric filmseparating said electrodes.

5. The capacitor of claim 3 wherein said conductive surface portion ofsaid rotor is a metal wire gauze.

6. The capacitor of claim 5 wherein said second electrode is an annularsegment, and said conductive surface portion of said rotor is an annularsegment of at least slightly greater length than said second electrodeand is disposed at substantially the same radial spacing as said secondelectrode.

7. The capacitor of claim 5 wherein said rotor includes a low frictionsurface portion substantially coplanar with said conductive surfaceportion thereof.

8. The capacitor of claim 3 wherein said rotor includes a flexibleforaminous conductive layer adapted to be urged into contact with saidsecond terminal and said island areas.

9. The contact of claim 8 wherein said conductive layer is a wire gauze,and said rotor includes a resilient support layer for urging said wiregauze into contact with said second terminal and said island areas.

10. The contact of claim 9 including a member of low friction materialdisposed adjacent said wire gauze so as to pro vide extended surfacearea of said contact.

